| 1. |
- Aquilante, Francesco, et al.
(författare)
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Low-cost evaluation of the exchange Fock matrix from Cholesky and density fitting representations of the electron repulsion integrals
- 2007
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 126:19, s. 194106-
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Tidskriftsartikel (refereegranskat)abstract
- The authors propose a new algorithm, “local K” (LK), for fast evaluation of the exchange Fock matrix in case the Cholesky decomposition of the electron repulsion integrals is used. The novelty lies in the fact that rigorous upper bounds to the contribution from each occupied orbital to the exchange Fock matrix are employed. By formulating these inequalities in terms of localized orbitals, the scaling of computing the exchange Fock matrix is reduced from quartic to quadratic with only negligible prescreening overhead and strict error control. Compared to the unscreened Cholesky algorithm, the computational saving is substantial for systems of medium and large sizes. By virtue of its general formulation, the LK algorithm can be used also within the class of methods that employ auxiliary basis set expansions for representing the electron repulsion integrals.
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| 2. |
- Boström, Jonas, et al.
(författare)
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Ab Initio Density Fitting : Accuracy Assessment of Auxiliary Basis Sets from Cholesky Decompositions
- 2009
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Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 5:6, s. 1545-1553
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Tidskriftsartikel (refereegranskat)abstract
- The accuracy of auxiliary basis sets derived by Cholesky decompositions of the electron repulsion integrals is assessed in a series of benchmarks on total ground state energies and dipole moments of a large test set of molecules. The test set includes molecules composed of atoms from the first three rows of the periodic table as well as transition metals. The accuracy of the auxiliary basis sets are tested for the 6-31 G**, correlation consistent, and atomic natural orbital basis sets at the Hartree-Fock, density functional theory, and second-order Moller-Plesset levels of theory. By decreasing the decomposition threshold, a hierarchy of auxiliary basis sets is obtained with accuracies ranging from that of standard auxiliary basis sets to that of conventional integral treatments.
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| 3. |
- De Vico, Luca, et al.
(författare)
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Chemiluminescence of 1,2-dioxetane. Reaction mechanism uncovered
- 2007
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 111:32, s. 8013-8019
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Tidskriftsartikel (refereegranskat)abstract
- The thermal decomposition of 1,2-dioxetane and the associated production of chemiluminescent products, model for a wide range of chemiluminescent reactions, has been studied at the multistate multiconfigurational second-order perturbation level of theory. This study is in qualitative and quantitative agreement with experimental observations with respect to the activation energy and the observed increase of triplet and singlet excited products as substituents are added to the parent molecule. The, previously incomplete, reaction mechanism of the chemiluminescence of 1,2-dioxetane is now rationalized and described as mainly due to a particular form of entropic trapping.
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| 4. |
- De Vico, Luca, et al.
(författare)
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New general tools for constrained geometry optimizations
- 2005
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Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 1:5, s. 1029-1037
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Tidskriftsartikel (refereegranskat)abstract
- A modification of the constrained geometry optimization method by Anglada and Bofill (Anglada, J. M.; Bofill, J. M. J. Comput. Chem. 1997, 18, 992-1003) is designed and implemented. The changes include the choice of projection, quasi-line-search, and the use of a Rational Function optimization approach rather than a reduced-restricted-quasi-Newton-Raphson method in the optimization step. Furthermore, we show how geometrical constrains can be implemented in an approach based on nonreclunclant curvilinear coordinates avoiding the inclusion of the constraints in the set of redundant coordinates used to define the internal coordinates. The behavior of the new implementation is demonstrated in geometry optimizations featuring single or multiple geometrical constraints (bond lengths, angles, etc.), optimizations on hyperspherical cross sections (as in the computation of steepest descent paths), and location of energy minima on the intersection subspace of two potential energy surfaces (i.e. minimum energy crossing points). In addition, a novel scheme to determine the crossing point geometrically nearest to a given molecular structure is proposed.
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| 5. |
- Devarajan, Ajitha, et al.
(författare)
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Role of Electronic Curve Crossing of Benzene S-1 State in the Photodissociation of Aryl Halides, Effect of Fluorination : RASSI-SO MS-CASPT2 Study
- 2009
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Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 109:9, s. 1962-1974
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Tidskriftsartikel (refereegranskat)abstract
- An ab initio study of the role of electronic curve crossing of benzene S-1 state in the photo dissociation dynamics of the iodobenzene and effect of fluorination is presented. Two dissociative life times observed in iodobenzene is attributed to the coupled repulsive potential energy curves of the low-lying n-sigma*, pi-sigma*, pi-pi* states. The direct channel is attributed to the alkyl like transition and the indirect channel is attributed to the mixing of the alkyl like transitions with the low lying benzene pi-pi* transitions. Fluorination of iodobenzene results in a substantial increase in the direct channel product. To analyze the possible role of electronic curve crossing of these transitions, potential energy curves of low-lying n-sigma*, pi-sigma*, pi-pi* states were studied including spin-orbit and relativistic effects using the Restricted Active Space state interaction multistate complete active space perturbation theory (RASSI-MS-CASPT2) method. Crossing behavior of spin-free and spin-orbit potential energy curves was analyzed for the role of the benzene S-1 state. Our results indicate the curve crossing region to be around 2.00-2.35 angstrom for both C6H5I and C6F5I. Analysis of effect of fluorination on the energies of states corresponding to benzene pi-pi* and n-sigma* transitions suggests an increase in the energy of benzene pi-pi* states and a decrease in the energy of the states corresponding to n-sigma* transitions. Increased spin-orbit gap, increased separation of the benzene S-1(pi-pi*) state and n-sigma* states in the region of curve crossing, lesser mixing of the pi-pi* and n-sigma* states, an order of magnitude decrease in the transition strength to the benzene singlet transition all contributed to the observed Substantial increase in the quantum yield of the direct channel product on fluorination of aryl halides.
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| 6. |
- Gaenko, Alexander V, et al.
(författare)
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Ab initio DFT study of Z-E isomerization pathways of N-benzylideneaniline
- 2007
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Ingår i: Theoretical Chemistry accounts. - : Springer Science and Business Media LLC. - 1432-881X .- 1432-2234. ; 118:1, s. 271-279
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Tidskriftsartikel (refereegranskat)abstract
- The ground state properties and absorption spectra of N-benzylideneaniline (NBA) have been studied at the density functional (DFT) and at the time-dependent density functional (TD-DFT) level of the theory. The equilibrium geometries of the E and Z isomers in the ground state and their vibrational frequencies have been computed. Furthermore, the excitation energies of the lowest excited singlet and triplet states and the potential energy curves along the torsion and the inversion isomerization coordinates were evaluated. The results are discussed in light of the available experimental data.
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| 7. |
- Holt, Asbjörn, et al.
(författare)
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The charge capacity of the chemical bond
- 2007
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 436:1-3, s. 297-301
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Tidskriftsartikel (refereegranskat)abstract
- A bond capacitance is introduced via the LoProp formalism, and the calculated bond capacitances reflects the properties of the corresponding chemical bond remarkably well. It is clearly possible to characterize the properties of the chemical bond with the help of the calculated bond capacitances.
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| 8. |
- Krogh, Jesper Wisborg, et al.
(författare)
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The prediction of the nuclear quadrupole splitting of Sn-119 Mossbauer spectroscopy data by scalar relativistic DFT calculations
- 2006
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Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 12:19, s. 5116-5121
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Tidskriftsartikel (refereegranskat)abstract
- The electric field gradient components for the tin nucleus of 34 tin compounds of experimentally known structures and Sn-119 Mossbauer spectroscopy parameters were computed at the scalar relativistic density functional theory level of approximation. The theoretical values of the electric field gradient components were used to determine a quantity, V, which is proportional to the nuclear quadrupole splitting parameter (Delta E). In a subsequent linear regression analysis the effective nuclear quadrupole moment, Q, was evaluated. The value of (11.9 +/- 0.1) fm(2) is a significant improvement over the non-relativistic result of (15.2 +/- 4.4) fm(2) and is in agreement with the experimental value of (10.9 +/- 0.8) fm(2). The average mean square error Delta E-calcd-Delta E-exptl = +/- 0.3 mm s(-1) is a factor of two smaller than in the non-relativistic case. Thus, the approach has a quality which provides accurate support for the structure interpretation by Sn-119 spectroscopy. It was noted that geometry optimization at the relativistic level does not significantly increase the quality of the results compared with non-relativistic optimized structures. The accuracy in the approach called on us to consider the singlet-triplet state nature of the electronic structure of one of the investigated compounds.
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| 9. |
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| 10. |
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| 11. |
- Liu, Fengyi, et al.
(författare)
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A CASSCF/CASPT2 approach to the decomposition of thiazole-substituted dioxetanone : Substitution effects and charge-transfer induced electron excitation
- 2009
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 484:1-3, s. 69-75
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Tidskriftsartikel (refereegranskat)abstract
- The gas-phase decompositions of a thiazole-substituted dioxetanone, in both the natural and anionic forms, were investigated theoretically in a CASSCF/CASPT2 study. The neutral conjugated thiazole (with or without a hydroxyl group) substitution on the dioxetanone has no evident effect on the dissociation; however, a subsequent deprotonation - invoking charge-transfer excitations from the thiazole to the dioxetanone moiety - will dramatically change the reaction mechanism from stepwise to concerted, and reduce the activation barrier by similar to 8 kcal mol(-1). These findings are helpful for the better understanding of dioxetanone chemiluminescence and the charge-transfer induced electron excitation in chemi- and bioluminescence processes.
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| 12. |
- Liu, Ya-Jun, et al.
(författare)
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Spin-orbit ab initio investigation of the ultraviolet photolysis of diiodomethane
- 2007
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Ingår i: ChemPhysChem. - : Wiley. - 1439-4235 .- 1439-7641. ; 8:6, s. 890-898
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Tidskriftsartikel (refereegranskat)abstract
- The UV photodissociation (<5 eV) of diiodomethane (CH(2)l(2)) is investigated by spin-orbit ab inito calculations. The experimentally observed photodissociation channels in the gas and condensed phases are clearly assigned by multi-state second-order multiconfigurational perturbation theory in conjunction with spin-orbit interaction through complete active space-state interaction potential energy curves. The calculated results indicate that the fast dissociations of the first two singlet states of CH(2)l(2) and CH(2)l-l lead to geminate-radical products, CH(2)l(+) +l(P-2(3/2)) or CH(2)l(+) + l*(P-2(1/2)). The recombination process from CH(2)l-l to CH(2)l(2) is explained by an isomerazation process and a secondary photodissociation reaction of CH(2)l-l. Finally, the study reveals that spin-orbits effects are sifnificant in the quantitative analysis of the electronic spectrum of the CH(2)l-l species.
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| 13. |
- Mastalerz, Remigius, et al.
(författare)
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Analytic high-order Douglas-Kroll-Hess electric field gradients
- 2007
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 127:7, s. 074105-
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Tidskriftsartikel (refereegranskat)abstract
- In this work we present a comprehensive study of analytical electric field gradients in hydrogen halides calculated within the high-order Douglas-Kroll-Hess (DKH) scalar-relativistic approach taking picture-change effects analytically into account. We demonstrate the technical feasibility and reliability of a high-order DKH unitary transformation for the property integrals. The convergence behavior of the DKH property expansion is discussed close to the basis set limit and conditions ensuring picture-change-corrected results are determined. Numerical results are presented, which show that the DKH property expansion converges rapidly toward the reference values provided by four-component methods. This shows that in closed-shell cases, the scalar-relativistic DKH(2,2) approach which is of second order in the external potential for both orbitals and property operator yields a remarkable accuracy. As a parameter-dependence-free high-order DKH model, we recommend DKH(4,3). Moreover, the effect of a finite-nucleus model, different parametrization schemes for the unitary matrices, and the reliability of standard basis sets are investigated.
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| 14. |
- Pedersen, Thomas Bondo, et al.
(författare)
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Density fitting with auxiliary basis sets from Cholesky decompositions
- 2009
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Ingår i: Theoretical Chemistry accounts. - : Springer Science and Business Media LLC. - 1432-881X .- 1432-2234. ; 124:1-2, s. 1-10
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Tidskriftsartikel (refereegranskat)abstract
- Recent progress in the use of Cholesky decomposition techniques within the density fitting approximation of two-electron integrals is reviewed with emphasis on the theoretical background. Special attention is paid to the fact that errors due to the density fitting approximation can be controlled by constructing auxiliary basis sets by means of Cholesky decomposition of either the entire or certain subblocks of the molecular two-electron integral matrix. Finally, the prospects of trivial linear-scaling calculation of fitting coefficients in the Cholesky decomposition-based density fitting scheme are outlined.
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| 15. |
- Raab, Juraj, et al.
(författare)
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A combined experimental and theoretical study of uranium polyhydrides with new evidence for the large complex UH4(H-2)(6)
- 2007
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 111:28, s. 6383-6387
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Tidskriftsartikel (refereegranskat)abstract
- Several monouranium and diuranium polyhydride molecules were investigated using quantum chemical methods. The infrared spectra of uranium and hydrogen reaction products in condensed neon and pure hydrogen were measured and compared with previous argon matrix frequencies. The calculated molecular structures and vibrational frequencies were used to identify the species present in the matrix. Major new absorptions were observed and compared with the previous argon matrix study. Spectroscopic evidence was obtained for the novel complex, UH4(H-2)(6), which has potential interest as a metal hydride with a large number of hydrogen atoms bound to uranium. Our calculations show that the series of complexes UH4(H-2)(1,2,4,6) are stable.
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| 16. |
- Roos, Björn, et al.
(författare)
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New relativistic ANO basis sets for actinide atoms
- 2005
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 409:4-6, s. 295-299
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Tidskriftsartikel (refereegranskat)abstract
- New basis sets of the atomic natural orbital (ANO) type have been developed for the actinide atoms Ac-Cm. The ANOs have been obtained from the average density matrix of the ground and lowest excited states of the atom, the positive ions, and the atom in a electric field. Scalar relativistic effects are included through the use of a Douglas-Kroll-Hess Hamiltonian. Multiconfigurational wave functions have been used with dynamic correlation included using second order perturbation theory (CASSCF/CASPT2). The basis sets are applied in calculations of ionization energies and some excitation energies. Computed ionization energies have an accuracy better than 0.2 eV in most cases. The lowest multiplet levels have been computed. These calculations include spin-orbit coupling using a variation-perturbation approach. The atomic polarizability of the spherically symmetric americium atom has been computed to be 116 au(3).
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| 17. |
- Roos, Björn, et al.
(författare)
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New relativistic ANO basis sets for transition metal atoms
- 2005
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 109:29, s. 6575-6579
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Tidskriftsartikel (refereegranskat)abstract
- New basis sets of the atomic natural orbital (ANO) type have been developed for the first, second, and third row transition metal atoms. The ANOs have been obtained from the average density matrix of the ground and lowest excited states of the atom, the positive and negative ions, and the atom in an electric field. Scalar relativistic effects are included through the use of a Douglas-Kroll-Hess Hamiltonian. Multiconfigurational wave functions have been used with dynamic correlation included using second order perturbation theory (CASSCF/CASPT2). The basis sets are applied in calculations of ionization energies, electron affinities, and excitation energies for all atoms and polarizabilities for spherically symmetric atoms. These calculations include spin-orbit coupling using a variation-perturbation approach. Computed ionization energies have an accuracy better than 0.2 eV in most cases. The accuracy of computed electron affinities is the same except in cases where the experimental values are smaller than 0.5 eV. Accurate results are obtained for the polarizabilities of atoms with spherical symmetry. Multiplet levels are presented for some of the third row transition metals.
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| 18. |
- Serrano-Andres, Luis, et al.
(författare)
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Computation of conical intersections by using perturbation techniques
- 2005
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 122:10, s. 104107-
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Tidskriftsartikel (refereegranskat)abstract
- Multiconfigurational second-order perturbation theory, both in its single-state multiconfigurational second-order perturbation theory (CASPT2) and multistate (MS-CASPT2) formulations, is used to search for minima on the crossing seams between different potential energy hypersurfaces of electronic states in several molecular systems. The performance of the procedures is tested and discussed, focusing on the problem of the nonorthogonality of the single-state perturbative solutions. In different cases the obtained structures and energy differences are compared with available complete active space self-consistent field and multireference configuration interaction solutions. Calculations on different state crossings in LiF, formaldehyde, the ethene dimer, and the penta-2,4-dieniminium cation illustrate the discussions. Practical procedures to validate the CASPT2 solutions in polyatomic systems are explored, while it is shown that the application of the MS-CASPT2 procedure is not straightforward and requires a careful analysis of the stability of the results with the quality of the reference wave functions, that is, the size of the active space.
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